IDOC creation when PO create or PO change

hi all,

i would like to know the steps for generating IDOC when PO create or PO change...

i have done all the required configuration  in WE21...and MN04....etc

but  when i create PO or change PO idoc is not generating....what should i do....

can anyone provide me the steps to IDOC......

thanks..

surya